Aluminum has been the principal material used in aircraft and aerospace construction for the past 60 years. With the growing interest to construct more efficient aircraft, manufacturers are designing more components out of light-weight composite materials. Current composite structures include the fuselage, wing skins, engine nacelles, control surfaces, wing tips (winglets), and even rotary blades on helicopters and wind turbines. Composites, however, are poor conductors of electrical current. Without proper protection, composite materials are susceptible to severe damage in the event of a lightning strike. To date, aircraft manufacturers have used aluminum or copper expanded foils or woven wire mesh incorporated into the surface of these composite structures to dissipate lightning strike energy and prevent damage to the composite material.
Of the two approaches, expanded metal foils have become the industry standard and are superior to woven wire as they do not unravel or have loose strands that may become problematic during processing into a pre-preg material or when conducting a dry lay-up as part of the composite manufacturing process. The homogenous design of expanded metal foils also ensures uncompromised conductivity even when forming the material into a variety of shapes and contours and it provides a smooth surface on the end product. Expanded metal foils used in this application must be manufactured with tight tolerances to meet a specific weight, open area, and conductivity requirements.
Aircraft manufactures use design guidelines, such as those set forth by SAE International in its Aerospace Recommended Practice (ARP) 5414, which defines lightning strike zones (areas of the aircraft more susceptible to lightning strikes (e.g. Zone 1A, 1B). It also provides required electrical withstand capabilities for such strike zones. For example, often materials are required to have the ability to withstand a Zone 1A strike of 200,000 amps. For expanded foils, due to the limitations of the expansion process, the thinnest material possible to meet this criteria to date has been produced using 42 micron foils. The weight of this material is 175 grams per square meter, the resistivity is 3.6 milliohms per square, and the foil has 56% open area. A way to characterize the performance of foils in this application is to assess the foil's weight to conductivity ratio, with conductivity being the inverse of resistivity and represented in gram-ohms per square. For the above expanded foil, its weight to conductivity ratio is 0.63 gr-ohms.
Aircraft manufacturers are always looking for ways to increase efficiency, reduce costs, improve fuel economy, and reduce the amount of CO2 emissions. One clear way to achieve these objectives is to reduce aircraft weight. By reducing the weight of the composite material, the overall weight of the aircraft may be reduced; however, the conductivity criteria required for specific strike zones per SAE ARP5414 must still be satisfied. Thus, it would be desirable to produce thinner and lighter foils, which still meet the required electrical withstand capabilities. Another way of stating this is that it would be very desirable to minimize the foil weight to conductivity ratio.